Commercial AM broadcasting has only weakly embraced the broadcasting of stereo signals. For a short period of time there were many commercial AM stations at least experimenting with the concept and at one point in time there were numerous AM receivers produced capable of decoding broadcast AM stereo signals. Several automobile manufactures provided in-dash receivers with that capability.
Many types of AM stereo modulation systems are known in the art. These include quadrature modulation in which a first signal modulates a carrier signal, and a second signal modulates a carrier signaling a 90· phase difference from the first carrier signal. The second, or quadrature, modulator is a suppressed carrier type, such that only the sidebands of the modulated signal remain. The quadrature modulated side-bands are added to the output signal of the first modulator to produce quadrature modulation.
For AM stereo transmission, the input of the first transmitter is the main (L+R) signal and the input of the second transmitter is the stereo (L−R) signal. This is pure quadrature” modulation and as such, it is not compatible with current AM receivers.
A quadrature modulation method which became the de facto standard for stereo AM broadcasting was developed by Motorola Corporation in the early 1980s. This system was invented in 1977 by Norman Parker, Francis Hilbert, and Yoshio Sakaie, and published in an IEEE journal. Using circuitry developed by Motorola Incorporated, this modulation system used quadrature amplitude modulation (QAM) to encode the stereo separation signal. This extra signal is then stripped down in such a way that it is compatible with the envelope detector of older receivers (hence the name C-QUAM (i.e., Compatible QUadrature Amplitude Modulation).
While many implementations of the C-QUAM system have provided, each suffers from its own unique deficiencies. Typically, C-QAUM encoders utilize many stages (typically adders, limiters, modulators, etc.) to produce the desired C-Quam compatible AM stereo signal.
While the need for stereo AM in the commercial broadcasting environment has all but disappeared, the need for AM stereo continues in several small but important environments. Museums and individuals have need for a simple, inexpensive way to provide low power AM signals for local broadcast to monophonic or stereophonic vintage radios in their collections.
It would, therefore, be advantageous to provide a simple, low-cost system for providing both monophonic and stereophonic low power AM C-QUAM compatible broadcast signals for use by museums and individual collectors of vintage and antique AM radio receivers.